Printed circuit boards electrically connecting electronic components or electronic devices require functionality to realize matching of characteristic impedance, that is, an alternating electric current characteristic, to improve high speed transmission and high frequency characteristics, to reduce unnecessary emissions, and the like, as transmission signals are speeded up. Accordingly, the printed circuit boards are configured from striplines and microstrip lines, and with the increase of parallel signal lines, multilayering is necessary.
For multilayer printed circuit boards for high speed transmission, composed of striplines, various measures have, up to now, been devised for crosstalk problems, and problems of signal attenuation or reflection due to minute loads on transmission paths formed of through-hole vias.
For example, in a circuit board for high speed signals, having a multilayer structure with a signal layer and a ground (GND) layer, there is an invention that reduces reflection of signals in the vicinity of a via land, and in particular, improves transmission characteristics of high speed signals, by arranging a ground pattern so as to surround the via land in a signal layer that joins signal lines in two different layers by a via (for example, Japanese Unexamined Utility Model Application, First Publication No. Hei 5-93080, referred to below as Patent Document 1).
Furthermore, as a printed circuit board having a multilayer wiring structure suitable for high speed signal transmission, including geometrically shaped forms for signal wiring and power source/ground system conductors, an invention composed of a multilayer printed circuit board is disclosed, having an electrical potential supply system layer, formed of plural parallel strip conductors each given a prescribed electrical potential, and a signal wiring layer configured by laminating the electrical potential supply system layer separated by an insulating layer, with wiring parallel to the parallel strip conductors (for example, Japanese Unexamined Patent Application, First Publication No. Hei 8-46078, referred to below as Patent Document 2).
In the abovementioned Patent Document 2, if the conductor width in a pattern of the parallel strip conductors is less than or equal to twice the signal wiring, and the pattern pitch is less than or equal to three times the signal wiring width, an optimal pattern can be obtained with minimal propagation delay and little waveform distortion.
In addition, as a printed circuit board in which crosstalk between signal lines and signal interference is reduced, with plural paths with fixed impedance arrayed on the printed circuit board, an invention composed of a multilayer printed circuit board is disclosed, being provided with a ground pattern area arranged between each adjacent line, and a via hole arranged in the ground pattern area, so as to conduct electricity to this ground pattern area and other ground pattern areas, each adjacent line being shielded (for example, Japanese Unexamined Patent Application, First Publication No. Hei 8-242078, referred to below as Patent Document 3).
Similar to the abovementioned Patent Document 3, as a circuit board for a high frequency circuit in which high frequency wiring is easy, and unnecessary electromagnetic coupling between transmission paths is reduced, in a multilayer circuit board for a high frequency circuit with stripline configuration transmission paths, including plural layers with signal layers between two grounded conducting layers, an invention is disclosed composed of the multilayer circuit board for a high frequency circuit in which signal lines and ground lines are laid out alternately in the signal layer, and the signal lines and the ground lines are laid out opposite each other, in adjacent signal layers, the ground lines having a width that is wider than the signal lines opposite (for example, Japanese Unexamined Patent Application, First Publication No. 2003-69239, referred to below as Patent Document 4).
In the abovementioned Patent Document 4, the effect of crosstalk between the signal lines and of electromagnetic noise from outside can be restrained and high density wiring can be realized. In addition, by providing a microstripline structure in an outer region, signal lines for passing signals and the like, with little crosstalk, can be provided. Furthermore, a stripline structure that does not have internal ground lines can be configured, and signals with little crosstalk can be passed in signal lines in the internal stripline structure.
Additionally, as a flexible board, an invention is disclosed composed of a flat shielded cable, having an insulating layer, formed from a liquid crystal polymer and integrally folded in two, signal wiring and ground wiring insulated and separated on mutually opposing faces of the insulating layer, that is folded (for example, folded in two) and arrayed integrally, a shield layer, integrally disposed on an external face of the insulating layer, for covering a region in which the signal wiring and the ground wiring are laid out, and an electrical conductor, passing through the insulating layer, for electrically connecting the ground wiring and the shield layer (for example, Japanese Unexamined Patent Application, First Publication No. 2001-135974, referred to below as Patent Document 5).
In the abovementioned Patent Document 5, by electrically connecting the shield layer and the ground wiring, with respect to the signal wiring, and in addition by the folding of the integrated shield layer, required sealing is realized. That is, the liquid crystal polymer that forms the insulating layer has good high frequency characteristics with a low dielectric constant, and exhibits stable functionality with little hygroscopicity, and together with characteristics that do not require high level machining accuracy, a flexible flat shielded cable with high reliability and low cost can be provided, so that it is possible to realize enhanced performance with the high frequency signal circuit.
Furthermore, as a flexible multilayer circuit board, an invention composed of a multilayer circuit board is disclosed in which the multilayer circuit board, formed of an organic material, built by laminating plural insulating layers in which wiring conductors, formed of metal foil, are arranged on at least one of the upper and lower surfaces, electrically connects the wiring conductors that sandwich, by being positioned above and below, the insulating layers, by a penetrating conductor formed in the insulating layers, the insulating layers being formed of a covering layer made of polyphenylene ether based organic matter on the upper and lower surfaces of a liquid crystal polymer layer, the liquid crystal polymer layer having an anisotropic dielectric property in which its dielectric constant is greatest in one direction among the directions parallel to its surface, and with a dielectric constant epsilon-X in the direction in which the dielectric constant is greatest, among directions parallel to the surface of the liquid crystal polymer layer, and with a dielectric constant epsilon-Y in a direction orthogonal to the direction in which the dielectric constant is greatest, among the directions parallel to the surface of the liquid crystal polymer layer, an anisotropic dielectric property degree, expressed as the ratio, epsilon-X/epsilon-Y, is between 1.2 and 2.0, (for example, Japanese Unexamined Patent Application, First Publication No. 2002-290055, referred to below as Patent Document 6).
In the abovementioned Patent Document 6, the insulating layer is formed of the covering layer made of the polyphenylene ether type organic matter on the surface of the liquid crystal polymer layer, so that minute through-holes can be provided by boring; as a result, the multilayer circuit board having high density wiring is possible, and in addition, the dielectric constants of the liquid crystal polymer layer and the covering layer formed from the polyphenylene ether based organic matter are low in a high frequency region, so that the multilayer circuit board with excellent transmission characteristics in the high frequency region is possible.